1. Field of Invention
The present invention relates to a sensing circuit in a memory, and more particularly, to a digital sensing circuit in a multi-stable memory device.
2. Description of Related Art
In recent years, bistable materials are often used in the fabrication of memory devices, switching devices and so on. The bistable material comprises an inorganic and an organic material. Furthermore, the bistable material can switch between a high and a low resistance state according to the applied voltage. It should be noted that a multi-state memory device could be produced when this type of bistable organic material is disposed between two electrodes and the multi-state memory device has great potential to become the next generation of non-volatile memory.
Compared with a silicon-based device, an organic device fabricated from an organic material has the advantages of a better extensibility and bending capacity. Because the organic material can be coated on virtually any types of surfaces, the idea of forming an organic memory array on a flexible plastic substrate can be realized. Furthermore, the organic material can be fabricated and processed after all the silicon processes have been completed so that the entire processing operation is simplified. Due to the aforementioned advantages and properties, many types of printing manufacturing processes are likely to be developed for mass-producing the organic devices, thereby substantially reducing the production cost of the device and widening its applications.
FIG. 1 is a graph showing the ideal operating curve of an organic memory cell inside an organic memory. The organic memory cell is fabricated using an organic material. As shown in FIG. 1, the organic memory cell has at least a bistable characteristic. That is, the organic memory cell can station in a high resistance state or a low resistance state. When the organic memory cell is in a high resistance state, the relation between the conduction current and the biased voltage follows the indicated path 110. Thus, if a biased voltage VR is applied to the organic memory cell when the organic memory cell is in the high resistance state, then the conduction current flowing through the organic memory cell will be I0. When the applied biased voltage exceeds VT1, the organic memory cell will change from a high resistance state to a low resistance state. Thereafter, the relation between the biased voltage and the conduction current will follow the path 120. Hence, if the applied biased voltage is VR when the organic memory cell is in the low resistance state, then the conduction current flowing through the organic memory cell is I1, where I1>>I0. After that, if the applied biased voltage is lower than VT0, then the organic memory cell will revert from a low resistance state back to a high resistance state. It should be noted, however, that the characteristic curve in FIG. 1 is highly idealized. In general, if a different organic material is used in the organic memory cell, the characteristic curve may differ slightly. Yet, all in all, the basic characteristics of the organic memory do not change considerably from the ideal characteristic curve shown in FIG. 1.
Accordingly, a memory fabricated using a bistable organic material can have a greater bending capability such that it can be used in an elastic and bendable system with flexible electronic devices. More specifically, the organic memory has a low production cost so that it can become one of the most important electronic memory devices in the world. Hence, there is an urgent need for developing a practical and complete organic memory. In particular, it is extremely important to develop a digital sensing circuit with simple design and layout, high yield rate, and easy production.